In order to carry out transient characterization of gas discharged from internal combustion engines (hereinafter call "exhaust gas"), the exhaust gas flow rate must be measured in real time. A trace method is one technique used to continuously measure the exhaust gas flow rate. This trace method introduces inert gas, for example, helium gas, which does not react with the components in the exhaust gas to the exhaust passage linked to the internal combustion engine. The method then measures the helium gas concentration with a trace gas analyzer connected to the gas sampling passage connected to the exhaust passage. The exhaust gas flow rate is then determined in real time by dividing the introducing rate of the helium gas by the concentration of the helium gas.
Examples of equipment for measuring the exhaust gas flow rate operating on the above-mentioned conventional measuring principle include that disclosed in Japanese non-examined Patent Publication No. Hei 8-15253. FIG. 22 schematically shows conventional engine exhaust gas flow rate measuring equipment disclosed in this patent publication. Numeral 71 designates an engine, numeral 72, a compressed gas cylinder for introducing helium gas as inert gas into this engine 71, and numeral 73, a pressure reducing valve. Numeral 74 designates an exhaust passage linked to the engine 71. Numeral 75 designates a gas sampling passage branched and connected to the exhaust passage 74 at the upstream side, which is equipped with a filter 76 and a suction pump 77, and joined and connected to the exhaust passage 74 on the downstream side. Numeral 78 designates a trace gas analyzer connected to the gas sampling passage 75 via a connecting member 79.
The trace gas analyzer 78 may be a quadruple mass spectrometer, sector field mass spectrometer, or similar device, but since these analyzers have a high vacuum inside, microleakage orifice or variable leak valve (VLV) is used for the connecting member 79 in connecting to the gas sampling passage 75 to which the filter 76, the suction pump 77, etc. are installed.
In measuring the exhaust gas flow rate, for example, helium gas must be introduced as a trace gas into the exhaust pipe 74a linked to the engine 71. But conventionally, as shown in FIG. 23, a pipe 75 comprising of, for example, tetrafluorethylene resin which is strong to, for example, exhaust gas G and can withstand comparatively high temperature, is inserted and connected to cross nearly at right angles to the direction in which the exhaust gas G flows. Helium gas TG is introduced as trace gas to the exhaust pipe 74a in which exhaust gas G flows via this pipe 75. However, in the above-mentioned configuration a number of problems exist as described below, and the measurement accuracy of the exhaust gas flow rate is not always satisfactory.
As the microleakage orifice of VLV has a large inside dead volume, when a plurality of other gas analyzers are connected to the gas sampling passage 75 and exhaust gas components such as CO, CO.sub.2, NO.sub.x, HC, etc. are analyzed with these gas analyzers, lag time is generated in the trace gas analyzer 78 and the gas analyzer, and the output timing must be adjusted in both analyzers.
As the inside of the trace gas analyzer 78 is originally of high vacuum, the sensitivity varies in accordance with the gas component ratio in the exhaust gas. That is, when the trace gas analyzer 78 has its temperature adjusted to a specified level, helium gas is introduced while being mixed in the exhaust gas at a specified concentration via the connecting member 79. In the continuous measurement of exhaust gas discharged from the internal combustion engine such as automobile engines, if the exhaust gas component to be measured suddenly changes, the difference is generated in pressure inside the trace gas analyzer 78 due to the difference of viscosity depending on this exhaust gas component. If the pressure, volume, and temperature inside the trace gas analyzer 78 are denoted by P, V (constant), and T (constant), then the equation PV=nRT (n: molecular number of helium gas and R: constant) holds for the helium gas. But when the pressure change AP is, for example, positive, since P is proportional to n in the above equation, the introducing amount of helium gas increases, and the reading of the helium gas in the exhaust gas becomes higher than the actual value. Also, the low exhaust gas flow rate is indicated. On the contrary, if the pressure variation is negative, the helium introducing volume decreases in proportion to this variation, and then the reading of helium gas in the exhaust gas becomes lower than the actual value. And for exhaust gas flow rate, a higher value is obtained.
In FIG. 23, while the inside diameter of exhaust pipe 74a is as large as 100 mm, that of the pipe 75 for introducing helium gas is about 4 mm. As pipe 75 is inserted in such a manner to simply cross at right angles with the flowing direction of exhaust gas with respect to the exhaust gas 74a, mixing of the exhaust gas G from the engine with helium gas TG does not always take place satisfactorily. Consequently, errors occur in the helium gas concentration measurement results by the trace gas analyzer 78, and there has been an inconvenience in that the measurement accuracy of the exhaust gas flow rate is not always satisfactory.
On the other hand, with respect to the sensitivity calibration method, conventionally pure nitrogen gas (N.sub.2) is used for zero gas. At the same time, a mixture of several tens to several thousands ppm of helium gas is added with pure N.sub.2 as base for span gas which is used for zero calibration and span calibration of the trace gas analyzer. Problems analogous to those described above exist with this configuration.
As no consideration is given to carbon dioxide (CO.sub.2) contained in a large quantity next to N.sub.2 in the exhaust gas and as calibration was carried out, the sensitivity change and the desired sensitivity calibration are unable to be carried out. With regard to the calculation of continuous mass emission rate from a car, the measured flow rate and a gas concentration for each gaseous constituents must be multiplied. In the conventional way, since the flow rate is always measured as a whole, gaseous constituents and the gas concentration is typically dehumidified concentration, either the measured flow rate must be converted to dehumidified concentration or the gas concentration must be converted to pre-humidified concentration with mathematical way. The conversion generates additional source of error in getting mass emission due to the water vapor concentration has to be assumed on the perfect combustion in the engine.
As components of exhaust gas discharged from motorized vehicles such as automobiles varies with driving modes, the flow rate of the exhaust gas must be measured in real time in accordance with each driving mode. For example, as shown in FIG. 26, a conventional dilution analysis process has been adopted in which the exhaust gas in each driving mode is diluted by atmospheric gas so that the diluted exhaust gas has a constant flow rate. The diluted gas is introduced into a gas component analyzer c, and the discharge rate of each specific component is determined.
A modal mass analysis method using a dilution analysis process is known as one method for determining the discharge rate of the specific component. In this analysis method, let the flow rate of the exhaust gas sucked into the sampling passage d for performing a concentration measurement be Q.sub.A (which is a constant); let a dilution air rate measured by an ultrasonic flow meter .function. at the dilution air inflow passage e be Q.sub.D (t); and let the total suction flow rate by the constant flow rate sampler CVS be Q.sub.M (which is a constant). Then the exhaust gas flow rate Q.sub.WE (t) (which contains moisture) discharged from the specimen vehicle b can be found from the following arithmetic expression: EQU Q.sub.WE (t)=Q.sub.A +Q.sub.M -Q.sub.D (t) 3
On the other hand, let the concentration of the components to be measured in the exhaust gas measured by the gas concentration analyzer a be C.sub.WE (t). Then the exhaust volume (mass) M(t) of the components to be measured in the exhaust gas can be found by the following arithmetic expression: EQU M(t)=.rho..times.C.sub.WE (t).times.Q.sub.WE (t) 4
for each traveling mode. This kind of arithmetic method composed of Eq. 3 and Eq. 4 is the modal mass analysis method applied to the dilution analysis process. In FIG. 26 reference character g designates a heat exchanger; h, a constant flow-rate venturi pipe; i, a suction pump; j, a dehumidifier; and k, a suction pump.
Consequently, in the modal mass analysis method by the dilution analysis process described above, when a component which is the same as that to be measured in the exhaust gas is present in the atmosphere, there is a possibility in that the measurement results are influenced by this presence, and it is difficult to generate the limit in the measuring accuracy.
Even if a large flow-rate air purifier is used for dilution, it is obviously disadvantageous for the analyzer to analyze by further diluting the measured component if the measured component is at a low concentration. In addition, a great amount of investment is required for the equipment for purifying air and for the CVS (constant-volume sampler) equipment for sucking diluting air, resulting in a high cost for the overall analysis equipment, as well as an increased size.
On the other hand, in the computation of Eq. 4 for determining the mass emission rate M(t) of the component to be measured for each driving mode, the value containing the moisture, that is, the wet-based value, is used for the concentration C.sub.WE (t). But because the exhaust gas with moisture removed by the dehumidifier j is introduced to the gas analyzer c, the wet-based concentration C.sub.WE (t) is unable to be directly detected by the gas analyzer c.
Therefore, for convenience, a dry-based (i.e., free of moisture) concentration C.sub.DE (t) detected by the gas analyzer c is converted into the wet-based concentration C.sub.WE (t) separately, and changed in Eq. 4. For example, let the moisture content (i.e., the moisture content removed by the dehumidifier j) contained in the exhaust gas be C.sub.H20 (t). The wet-based concentration C.sub.WE (t) can then be determined by the following conversion equation: EQU C.sub.WE (t)=C.sub.DE (t).times.[1-C.sub.H20 (t)] 5
The moisture content C.sub.H20 (t) is C.sub.H20 (t)=1/10 (0.10) when the flow rate Q.sub.WE (t) of the exhaust gas is supposed to be 1, for example, when 10% moisture is contained in the total exhaust gas.
Consequently, the value of the moisture content C.sub.H20 (t) is unable to be actually measured, and a value considered empirically adequate (assumed value) is used. But because the actual moisture content C.sub.H20 (t) varies with fuels and measurement conditions, there are cases in which the wet-based concentration C.sub.WE (t) determined by Eq. 5 differs from the actual value. As a result, the value of the mass emission rate M(t) of the component to be measured which is determined by Eq. 4 cannot be said to be accurate, thereby generating difficulty in reproducibility.
Under these circumstances, it is the main object of this invention to provide an exhaust gas analyzer which can accurately measure the exhaust rate of each specific component gas in automobile exhaust gas by driving modes in real time without using diluting air, and to provide a method for computing the exhaust rate thereof.